The present invention relates to a process for producing meta aramid filaments having improved quality and an enhanced productivity.
A meta aramid has a molecular skeleton consisting essentially of aromatic cyclic structures and thus has exhibit excellent heat resistance and flame resistance. Therefore, the meta aramid is employed for the uses of filaments, films and sheets My for which high heat resistance and flame resistance important.
Filaments of above-mentioned meta aramid (which may be referred to as meta-type armide hereinafter), for example, poly-meta-phenyleneisophthalamide filaments, are generally produced by one of the following two industrial methods.
Namely, in the first method, a poly-meta-phenyleneisophthalamide solution is produced by low temperature solution polymerization of meta-phenylenediamine with isophthalic acid chloride in N,N-dimethylacetamide; hydrochloric acid dissolved, as a by-product, in the above-mentioned solution is neutralized with calcium hydroxide; and the polymer solution comprising calcium chloride produced by the above-mentioned neutralization and dissolved in the solution is directly subjected to a dry-spinning procedure, as disclosed in Japanese Examined Patent Publication No. 35-14,399 and U.S. Pat. No. 3,360,595. In this method, the content of calcium chloride contained in the polymer solution is about 45% by weight, based on the weight of the polyamide.
In the second method, meta-phenylenediamine is reacted with isophthalic acid chloride in an organic solvent, for example, tetrahydrofuran, which is not a good solvent for a meta-type aramid produced from the reaction; the reaction system is brought into contact with an aqueous solution system containing an inorganic acid-acceptor and water-soluble neutral salt, to cause the reaction to be completed; the resultant poly-meta-phenyleneisophthalamide produced in a powder form is isolated (as shown in Japanese Examined Patent Publication No. 47-10,863); the polymer powder is dissolved in an amide-type solvent; and the resultant solution is subjected to a wet-spinning procedure using an inorganic salt-containing aqueous coagulation bath, as shown in Japanese Examined Patent Publication No. 48-17,551.
The first method is advantageous in that the polymer solution can be used as a spinning solution without isolating the polymer, but is disadvantageous in that the spinning procedure is carried out in a dry-spinning method using an amide-type solvent having a high boiling temperature, and thus the energy cost for the spinning procedure is high, and when the number of spinning holes per spinneret is increased, the stability of the spinning procedure is significantly decreased. When the polymer solution is subjected to a wet-spinning procedure using an aqueous coagulation liquid, to solve the above-mentioned problems, almost all of the resultant filaments are devitrificated fibers having a low mechanical strength. Therefore, this wet-spinning method has not yet industrially utilized.
The second method is advantageous in that the problems of the first method do not occur, but is disadvantageous in that the solvent for the polymerization system is different from the solvent for the spinning system, a procedure for re-dissolving the polymer which has been isolated from the polymerization system is necessary and, to obtain a stable solution of the re-dissolved polymer, specific care and an accurate control for the re-dissolving procedure are necessary, as disclosed in Japanese Examined Patent Publication No. 48-4,661.
To solve the above-mentioned problems, currently various attempts have been made to obtain meta-type aramid filaments having a high mechanical strength without loss of clarity of the filaments by wet-spinning a meta-type aramid polymer solution prepared by a solution polymerization.
For example, Japanese Unexamined Patent Publication No. 10-88,421 and No. 10-53,920 provide a process in which a meta aramid solution containing a salt, at a concentration of 3% by weight or more, is used as a spinning solution; the spinning solution is wet-spun in an aqueous coagulation bath containing a solvent and a salt each in a specific range of concentration, at a specific range of temperature; the spun filaments are treated in a condition-controlling liquid having specific ranges of solvent concentration and salt concentration at a specific temperate range; and the condition-controlled filaments are drawn in an aqueous drawing bath having specific ranges of solvent concentration and a specific salt concentration at a specific temperature range. It is sure that the above-mentioned process enables filaments having excellent mechanical properties to be obtained with a reduced generation of voids. However, this process has a problem that the productivity of the filaments is lower than that of the process in which a spinning solution containing substantially no inorganic salt is used.
As another process, Japanese Unexamined Patent Publications No. 8-74,121 and No. 8-74,122 disclose a process in which an aromatic diamine is reacted with an aromatic dicarboxylic acid chloride in an amide-type solvent; the resultant meta aramid solution containing hydrochloric acid produced as a by-product is neutralized with calcium hydroxide, calcium oxide or calcium carbonate; the resultant neutralized solution containing an inorganic salt in a high concentration is mixed with specific amounts of an amide-type solvent and water; and the resultant liquid containing the polymer and water each in a specific range of content is employed as a spinning solution for a wet spinning procedure. When the above-mentioned process is carried out by using an aqueous coagulating liquid, the coagulating agent is prevented from penetrating into the coagulated filaments, and thus filaments having a low void content and a high transparency can be produced with a high stability. The filaments have a high drawability and thus drawn filaments having excellent mechanical properties can be obtained. This process, however, has such a problem that when the number of extruding holes of the spinneret is large, for example, 1,000 or 30,000 per spinneret, the wet-spinning with a high stability is difficult, and thus the production of aramid filaments having excellent quality with a high productivity cannot be carried out.
On another hand, as processes for wet-spinning a spinning solution having a low content of the inorganic salts, various processes in which hydrogen chloride produced by the polymerization procedure is neutralized with a specific neutralizing agent which causes a salt produced by the neutralization reaction to be insoluble in the solvent for the polymerization. However, when ammonia gas is used as shown in Japanese Unexamined Patent Publication No. 35-14,399 and Japanese Unexamined Patent Publication No. 49-129,096, it is difficult to completely remove the resultant insoluble ammonium chloride salt from the neutralized intermediate product, and thus, a problem that not only the spinning nozzles are readily soiled, but also that the stability of the wet-spinning procedure is insufficient, occurs. In the processes using a solid neutralizing agent in the form of granules, for example, sodium carbonate or sodium hydroxide, since the neutralization is carried out in the manner of a solid/liquid reaction, and the resultant neutralized salt is insoluble in the solvent and thus deposited on the surfaces of the neutralizing agent granules, the neutralization cannot be completely effected.
Also, in the neutralization procedure using hydrogen solidum carbonate in the state of a solid, as a neutralizing agent, carbon dioxide gas is generated during the neutralization reaction to remove the resultant neutralized salt from the surfaces of the solid neutralizing agent granules and to improve the efficiency of the neutralization procedure. However, the neutralization is imperfectly effected, and the generated gas causes bubbles in the resultant polymer solution and the spinnability of the polymer solution is reduced.
An object of the present invention is to provide a process, for producing meta aramid filaments, in which process high quality filaments can be produced by a wet-spinning procedure and the productivity of the filaments can be greatly enhanced.
Another object of the present invention is to provide a process, for producing meta aramid filaments, in which process hydrogen chloride produced as a by-product in a polymerization procedure can be neutralized with a high efficiency, the by-product salt can be easily removed, and thus the resultant polymer solution can be easily employed, after only a filtration treatment, for a wet-forming procedure, for example, as a spinning solution for a wet-spinning procedure, and by which process, high quality filaments can be produced by a wet-spinning procedure and the productivity of the filaments can be greatly enhanced.
The above-mentioned objects can be attained by the process of the present invention for producing meta aramid filaments.
The process of the present invention for producing meta aramid filaments by preparing a polymer solution of a meta aramid by a solution polymerization method and producing meta aramid filaments from the polymer solution, comprises the series of steps (1) to (7) of:
(1) a polymerization step in which a meta aramid is prepared by reacting a aromatic meta-diamine with a aromatic meta-dicarboxylic acid chloride in a polar organic solvent;
(2) a neutralization step in which hydrogen chloride contained in the polymer solution obtained in the polymerization step (1) is neutralized with a neutralizing agent comprising an alkali metal hydroxide capable of producing a salt insoluble in the polymerization solvent when reacted with the hydrogen chloride;
(3) a filtering step in which the salt deposited from the polymer solution in the neutralization step (2) is removed by filtration;
(4) a spinning solution-preparation step in which the polymer solution obtained in the filtering step (3) is mixed with water and a polar organic amide solvent, to provide a spinning solution;
(5) a coagulation step in which the meta aramid-spinning solution obtained in the spinning solution-preparation step (4) is directly extruded in filamentary streams into an aqueous coagulation liquid to coagulate the extruded spinning solution streams into the form of filaments;
(6) a water-washing step in which the filaments delivered from the coagulation step (5) are immersed in water to remove the salts and the solvent from the filaments; and
(7) drawing and heat-treatment steps in which the filaments delivered from the water-washing step (6) are drawn and heat-treated.
In the process of the present invention for producing meta aramid filaments, preferably in the spinning solution-preparation step (4), the meta aramid solution is added to a polar organic solvent-water mixed liquid having a water content of 25% by weight or less.
In the process of the present invention for producing meta aramid filaments, preferably 85 molar % or more of the recurring units of the meta aramid obtained in the polymerization step (1) are meta-phenyleneisophthalamide units.
In the process of the present invention for producing meta aramid filaments, preferably the polar organic amide solvent used in the polymerization step (1) is N-methyl-2-pyrrolidone or N,N-dimethylacetamide.
In the process of the present invention for producing meta aramid filaments, the alkali metal hydroxide which reacts with hydrogen chloride in the neutralization step (2) to form the salt insoluble in the polymerization solvent is preferably sodium hydroxide or potassium hydroxide.
In the process of the present invention for producing meta aramid filaments, preferably in the neutralization step (2), the polymer solution is added with an aqueous solution of the alkali metal hydroxide.
In the process of the present invention for producing meta aramid filaments, the alkali metal hydroxide is preferably in a concentration of 20 to 70% in the aqueous alkali metal hydroxide solution.
The process of the present invention for producing meta aramid filaments optionally further comprises a step (8) for adding calcium chloride or lithium chloride to the copolymer solution, before the neutralization step.
In the process for producing meta aramid filaments, preferably in the neutralization step (2), the neutralizing agent contains at least one member selected from calcium hydroxide and lithium hydroxide, and 5 to 55 molar % of the total amount of hydrogen chloride contained in the polymer solution are neutralized with calcium hydroxide and/or lithium hydroxide.
In the process of the present invention for producing meta aramid filaments, the spinning solution preferably contains calcium chloride and/or lithium chloride in an amount of 3 to 25% by weight based on the weight of the meta aramid.
In the process of the present invention for producing meta aramid filaments, the spinning solution preferably contains the meta aramid in a concentration of 12 to 18% by weight, and water in a content of 25 to 70% by weight based on the weight of the meta aramid.
In the process of the present invention for producing meta aramid filaments, preferably, in the polymerization step (1), the organic polar solvent comprises at least one member selected from N-methyl-2-pyrrolidone, N,N-dimethylacetamide, N,N-dimethylformamide, dimethylsulfoxide, N-methylcaprolactame, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylurea.
In the process of the present invention, firstly a meta aramid is produced by the step (1) in which the meta-type aromatic amine and the aromatic meta-dicarboxylic acid halide, which will be exemplified below, are reacted with each other in a polar amide solvent. In this step, additional para-type aromatic compounds may be copolymerized as a copolymerization component.
As the aromatic meta-diamine as mentioned above, meta-phenylenediamine, 3,4xe2x80x2-diaminodiphenylether, and 3,4xe2x80x2-diaminodiphenylsulfone; and derivatives of the above-mentioned compounds having substituents such as halogen atoms and/or alkyl groups having 1 to 3 carbon atoms, attached to the aromatic cyclic structures thereof, for example, 2,4-toluylenediamine, 2,6-toluylenediamine, 2,4-diaminochlorobenzene, and 2,6-diaminochlorobenzene, may be employed. Particularly, preferably, meta-phenylenediamine or mixed diamines containing meta-phenylene diamine in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more are employed.
Also, as the aromatic meta-dicarboxylic acid halides, isophthalic acid halides, for example, isophthalic acid chloride and isophthalic acid bromide; and derivatives of the above-mentioned compounds having substituents, for example, halogen atoms and/or alkoxy groups having 1 to 3 carbon atoms, for example, 3-chloroisophthalic acid chloride and 3-methoxyisophthalic acid chloride, may be employed. Particularly, preferably, isophthalic acid chloride and mixed carboxylic acid halides containing isophthalic acid chloride in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more, are employed.
In the copolymerization components usable in combination with the diamines and the carboxylic acid halides, the comonomeric aromatic diamines include benzene derivatives such as para-phenylene diamine, 2,5-diaminochlorobenzene, 2,5-diaminobromobenzene and aminoanisidines; and 1,5-naphthylenediamine, 4,4xe2x80x2-diaminodiphenylether, 4,4,-diaminodiphenylketone, 4,4xe2x80x2-diaminodiphenylamine, 4,41-diaminodiphenylmethane. Also, the comonomeric aromatic dicarboxylic acid halides include terephthalic acid chloride, 1,4-naphthalenedicarboxylic acid chloride, 2,6-naphthalenedicarboxylic acid chloride, 4,4xe2x80x2-biphenyldicarboxylic acid chloride and 4,4xe2x80x2-biphenylethercarboxylic acid chloride.
If the copolymerization component is employed in too large an amount, the resultant meta aramid may exhibit a degraded property. Thus the copolymerization component is preferably used in an amount of 15 molar % or less, more preferably 10 molar % or less, based on the total molar amount of the acid components for the polyamide.
The preferable meta aramid for the present invention is a polyamide having recurring meta-phenylene isophthalamide units in a content of 85 molar % or more, more preferably 90 molar % or more, still more preferably 95 molar % or more, further preferably 100 molar % based on the total recurring units.
The organic polar solvents usable for the present invention include N-methyl-2-pyrrolidone (NMP), dimethylformamide (DMF), dimethylacetamide (DMAc), dimethylsulfoxide (DMSO), N-methylcaprolactame, and N,N,Nxe2x80x2,Nxe2x80x2-tetramethylurea. Particularly, NMP and DMAc are preferably employed.
The meta aramid obtained by the polymerization in step (1) preferably has a degree of polymerization corresponding to an intrinsic viscosity (IV) of 1.3 to 3.0, more preferably 1.7 to 2.2, determined by using a concentrated sulfuric acid as a solvent, at a temperature of 30xc2x0 C.
Since the polymerization solution obtained by the polymerization step contains hydrogen chloride produced as a by-product of the polymerization reaction, the polymerization solution is subjected to a neutralization treatment in step (2). The neutralizing agent is not necessarily limited to that of a specific type, as long as the salt produced by the neutralization reaction is a alkali metal compound insoluble in the polymerization medium (polar organic amide solvent). However, from economical view point, sodium hydroxide and potassium hydroxide, particularly sodium hydroxide, is preferably employed. Also, unless the object of the present invention is hindered, other neutralizing compounds may be employed. For example, lithium hydroxide, calcium hydroxide and calcium oxide, which are soluble in the polymerization solvent, may be employed together with the above-mentioned neutralizing agent.
In step (2), the alkali metal hydroxide may be added in the form of fine particles or in the state of a solid, or in the state of an aqueous solution. Generally, the aqueous solution of the alkali metal hydroxide is preferably employed to neutralize hydrogen chloride produced by the polymerization reaction with a high efficiency, and to enhance the neutralization processability and the filtering property of the neutralized salt in the filtration step (3) which will be explained hereinafter, and the concentration of the neutralizing agent in the aqueous solution is preferably 20 to 70% by weight more preferably 30 to 60% by weight.
If the concentration of the neutralizing agent in the aqueous solution is too high, the sodium hydroxide or potassium hydroxide are easily deposited as a solid during the neutralization reaction and thus the neutralization is difficult to uniformly effect.
The neutralized salt produced as a by-product of the neutralization reaction exhibits a low solubility in the organic solvent-containing polymerization solution and, thus, is substantially not dissolved in the polymerization solution and deposits as solid particles and, therefore, the neutralized salt is removed by filtration in step (3). There is no limitation to the type of the filtering system and, thus, conventional methods, for example, filtration or centrifugal separation, can be utilized. In this case, there is no limitation to the filtration system and to the filtration material and, thus, conventional filtration systems and materials and specifically designed systems and materials may be utilized.
In the filtration step, if the temperature of the neutralized solution is too high, the stability of the solution may decreased and the content of the salt in the solution may increase, and if the neutralized solution temperature is too low, the filtration property of the solution decreases. Thus, the filtration temperature is preferably in the range between 50 and 90xc2x0 C., particularly between 70 and 90xc2x0 C.
Then, into the meta aramid solution obtained in the filtering step (3), water and a polar organic amide solvent are added together, to prepare a spinning solution (step (4)). In this step, water is preferably added in the state of a solution of water in a content of 25% by weight in the organic amide solvent. In the spinning solution, the concentration of the polymer is preferably controlled to 12 to 18% by weight, particularly 15 to 17% by weight. Also, the content of water is preferably 25 to 70% by weight, more preferably 30 to 50% by weight, based on the weight of the above-mentioned polymer.
By preparing the spinning solution having the polymer concentration and the water content in the range as mentioned above, the step stability of the wet spinning step (5) which will be explained below is enhanced, and also, the transparency of the obtained filaments is increased, and the quality of the filaments is improved.
Also, the meta aramid-spinning solution obtained in the above-mentioned step (4) may contain therein calcium chloride and/or lithium chloride, for the purpose of enhancing the stability of the solution. There is no limitation to means for causing the salts to be contained in the spinning solution. For example, in a stage before the neutralization step, for example, in the polymerization step, calcium chloride and/or lithium chloride may be added to the polymerization reaction solution, or in a stage before or simultaneously with the neutralization step using sodium hydroxide and/or potassium hydroxide, a portion, (preferably 5 to 55 molar %) of hydrogen chloride produced in the polymerization step may be neutralized with calcium hydroxide and/or lithium hydroxide so that the resultant calcium chloride and/or lithium chloride produced as a by-product is dissolved and contained in the spinning solution.
The calcium chloride and/or lithium chloride is preferably contained in a content of 3 to 25% by weight, more preferably 10 to 20% by weight based on the weight of the meta aramid, in the spinning solution. By controlling the content of calcium chloride and/or lithium chloride to as mentioned above, when water is contained, the stability of the resultant spinning solution is enhanced and simultaneously the coagulating property of the polymer during the wet-spinning procedure is enhanced and thus aramid filaments having excellent quality can be obtained.
The meta aramid solution obtained in the spinning solution-preparation step (4) is fed to step (5) in which the polyamide solution is spun into polyamide filaments by using a wet spinning apparatus. Namely, the spinning solution is directly extruded into an aqueous coagulation liquid and coagulated into the form of solid filaments.
The coagulated polyamide filaments are taken up from the coagulation bath and are immersed in water-washing liquid in the water-washing step (6) (preferably in multistage water-washing step), to control the content of the solvent and the content of the salt remaining in the filaments, and then the polyamide filaments are passed through the draw-heat treating step (7) and thereafter are optionally contained in the form of a tow in a packing can or are wound up or are directly fed to a successive step or, optionally, are crimpled, cut at a desired fiber length and then subjected to a desired processing step.
The spinneret usable in step (5) of the process of the present invention may be a filament-spinneret having 50 to 1,000 spinning holes. When a staple fiber-spinneret having 1,000 to 30,000 spinning holes is used for the process of the present invention, the effect of the present invention can be realized to the maximum. Namely, the spinning solution has a high coagulation property, and therefore, even when the spinneret has a large number of spinning holes, the resultant filamentary streams of the spinning solution can be coagulated with a high stability in the aqueous coagulating bath. Generally the size of the spinning holes is 0.05 to 0.2 mm. Preferably, during the spinning procedure, the temperature of the meta aramid solution is in the range of from 50xc2x0 C. to 90xc2x0 C.
An aqueous coagulating solution for step (5) of the process of the present invention, a conventional aqueous inorganic salt solution can be employed. For example, an aqueous solution containing calcium chloride in a content of 34 to 42% by weight and NMP in a content of 5 to 10% by weight is preferably employed. When this type of the coagulating liquid is employed, the temperature of the liquid is preferably in the range of from 80 to 95xc2x0 C.
The rate of withdrawing the coagulated filaments from the aqueous coagulating bath is preferably in the range of from 5 to 25 m/minute. From the view point of enhancing the productivity, the withdrawing rate is more preferably in the range of from 10 to 25 m/minute. The time for immersing the filaments in the coagulating liquid is preferably in the range of from 1.0 to 11 seconds.
The filaments withdrawn from the coagulating liquid are fed into the drawing step (7) in hot water through the water-washing step (6). The water-washing step (6) is preferably carried out in a plurality of stages, for example, 3 to 12 stages. For example, the filaments withdrawn from the coagulating liquid are cooled to a temperature of 60xc2x0 C. or less, and then are introduced into a first water-washing bath at a temperature of 30xc2x0 C. or less. In the first water-washing bath, the content of the polar inorganic solvent, for example, NMP is preferably in the range of from 15 to 25% by weight, and the washing water is supplemented to the water-washing bath so that the content of the polar inorganic solvent is maintained at the above-mentioned level. In this case, the flow rate and the solvent content of the washing water for the supplementation are provisionally designed. The immersion time of the filaments in the first water-washing bath is preferably 8 to 30 seconds. Then the filaments are further washed in a second water-washing bath at a temperature of 30 to 85xc2x0 C. The amount of the washing water and the solvent content of the washing water to be supplemented into the second water-washing bath, and the immersion time of the filaments in the second water-washing bath are optionally designed so that the amount of the solvent remaining in the filaments delivered from the water-washing step becomes 15 to 25% by weight based on the weight of the polymer and the contents of calcium chloride and/or lithium chloride in the filaments become 0.5% by weight or less.
After the amount of the remaining solvent and the contents of calcium chloride and/or lithium chloride in the filaments are controlled respectively to desired levels or less in the water-washing step (6), the resultant filaments are preferably drawn at a draw ratio of 2.8 to 3.5 in the hot water drawing step (7), while the remaining solvent and salts are wash-removed from the filaments. To keep the drawing step in a good condition, the drawing operation is preferably carried out in a plurality of drawing stages, preferably two stages or more, more preferably three stages or more.
In the step (7), the drawn filaments are dried at a temperature of 100xc2x0 C. or more, and heat-treated by using heating rollers or sheeting plates at a temperature of 270 to 350xc2x0 C. optionally, the filaments may be further drawn on the heating plates.
The meta aramid filaments produced through the process as mentioned above are optionally packed in the form of a tow in a packing can or wound up or fed to successive steps or, optionally, are crimped, and cut to provide staple fibers, and then are subjected to successive processing procedures.